#include <bits/stdc++.h>
using namespace std;
#define int long long

void solve() {
    double n, c, h;
    cin >> n >> c >> h;
    vector<double> a(n + 1);
    for(int i = 0; i < n; i++) cin >> a[i];
    // sort(a.begin(), a.end());
    a[n] = INT_MAX;
    double ans = 0;
    for(int i = 0; i < n; i++) {
        double t = 0;
        if(a[i + 1] < a[i] + h) {
            t = ((double)1 - ((a[i + 1] - a[i]) / h)) * c;
        }
        // cout << t << ' ';
        double s = (c + t) * (min(h, a[i + 1] - a[i])) / 2; 
        // cout << s << endl;
        ans += s;
    }
    printf("%.8lf\n", ans);
}

signed main() {
    int t;
    cin >> t;
    while(t--) solve();
    return 0;
}